Oxidative injury, mediated by the toxic effects of reactive oxygen species (ROS), is implicated in the pathogenesis of many diseases including carcinogenesis, aging and inflammation. The lung is a major target for exogenous oxidants, such as smoke and air pollution, as well as for the endogenous ROS generated by inflammatory cells. In addition, patients succumbing to respiratory failure (e.g. adult respiratory distress syndrome) require supplemental oxygen therapy which further increases the oxidant burden of the lung. Aerobic organisms have developed antioxidant defenses to defend against oxidative stress. One such defense strategy is the up-regulation of various stress-response gene such as heme oxygenase-1 (HO-1), a ubiquitous mammalian enzyme. There has been recent evidence implicating HO-1 as a cytoprotective gene product given its marked induction with oxidant stress and its ability to decrease the pro-oxidant state of the cell, by degrading heme. The by-products of HO-1 activity, bilirubin and ferritin, also have anti-oxidant properties by scavenging radicals and sequestering highly-reactive free iron, respectively. We have observed HO-1 to be highly induced in vivo and in vitro after hyperoxia (95% 0/2). Furthermore, we found HO-1 induction in hyperoxic RAW 264.7 macrophage cells to be transcriptionally regulated and dependent on cooperation between the proximal promoter and a distal enhancer site. Functionally, studies with lung epithelial cells show HO-1 protects against hyperoxic death and preliminary survival studies with transgenic mice show HO-1 offers protection against hyperoxia in vivo as well. We hypothesize hyperoxia up-regulates HO-1 as a protective mechanism. We will examine the transcriptional regulation and functional significance of HO-1 in hyperoxia by addressing the following specific aims: 1) Determine the transcriptional regulation of HO-1 gene expression in response to hyperoxia. 2) Identify the upstream signal transduction pathway(s) involved in the activation of the HO-1 gene after hyperoxia. 3) Determine the functional role of HO-1 after hyperoxia in vitro following hyperoxia. 4) Determine the functional role of HO-1 after hyperoxia in vivo using HO- 1 transgenic and knock-out mice.